This invention relates to a novel bag making machine. More specifically, this invention relates to a machine for making bags of thermoplastic resin film and a control system which eliminates undesirable effects caused by interrupting the bag making procedure.
Thermoplastic resin bags, such as those used in supermarkets, are produced from a long, flat, tubular web of thermoplastic synthetic resin film. The tubular web is intermittently fed into a bag making machine where heat is transversely applied to the web in order to form a seal between the two sides of the web. A cutting edge intersects and transversely severs the web adjacent the seal in order to complete formation of a bag.
A common configuration for achieving the above mentioned sequence of operations includes seal or platen rollers which intermittently draw a web of thermoplastic film into the bag making machine. Upon cessation of advancement of the web, a hot press mechanism, including a heated fixed seal bar and a heated movable reciprocating seal bar, engages and melts the top and bottom web layers together. Upon cooling of the thermoplastic web, a seal is formed. The sealing bars are coated with an anti-adhesive material, such as Teflon, in order to prevent the weld bead of the thermoplastic web from adhering to the seal bars. Associated with the upper reciprocating seal bar is a cutting edge which transversely severs the thermoplastic web adjacent to the seal. The drive rollers then advance the web the length of a bag in order to repeat the sealing and severing cycle. In the past this process has been rapidly repeated at a rate of approximately 100 to 120 cycles per minute.
Interruption of the bag making procedure necessary for reasons including removal of plastic which adheres to the cutting edge, replacement of the thermoplastic resin web, change over operations, etc. Because heat radiates from the heated seal bars, the web must be positioned away from the seal bars during interruption of the bag making procedure so that the thermoplastic film does not undesirably melt and adhere to the seal bars.
As a countermeasure to the above problem, the intermittent feeder has conventionally been provided with another driving source such as an air cylinder in addition to a motor in order to move the web away from the seal bars and cutting edge. When the bag machine operation is stopped, the web is isolated from the seal bars and cutting edge by retracting the web in the backward direction by means of the air cylinder. When the machine operation is resumed, the web is returned to the original position by the air cylinder.
Problems with the above described machine configuration have been encountered, at least in part, due to the physical nature and inertial effects of pulleys and brake and clutch mechanisms, which do not insure precise timing. Timing is an extremely important component of a bag making machine because correlation of various machine operations is imperative to obtain desired results. When it is desirable to increase the rate of production up to, for example, 150 to 160 cycles per minute, problems of inertia and timing are exponentially exacerbated.
More recently the quality of the severing action was improved by heating the cutting edge. This allowed for quicker severing of the thermoplastic web because both heat and the physical action of the cutting edge were employed to sever the web. However, adherence of the thermoplastic web to the cutting edge presented a problem. This further concern was addressed by the conception of a seal bar mechanism in which the cutting edge was heated to a degree higher than that of the adjacent heated seal bar in order to allow combustion removal of any carbon by-product of the thermoplastic web, thereby encouraging self-actuated cleansing of the cutting edge and uninterrupted operation of a bag making machine. However, an elevated temperature of the cutting edge with respect to the seal bar adversely affects the Teflon coating on the seal bar. The integrity of the seal itself may also be affected because radiant heat emanating from the cutting edge tends to melt adjacent thermoplastic web.
Problems associated with radiant heat transfer from a superheated cutting edge were addressed by positioning the cutting edge away from the seal bar during periods of cycle repose. More specifically, the cutting edge was vertically recessed from the seal bars in order to allow air circulation between the two members. However, in order that the sealing and cutting operations occur substantially concurrently, and thereby encourage efficient machine operation, the cutting edge must advance and withdraw from the thermoplastic web at a rate greater that that of the movable heated seal bar. During any instance of interruption of the bag making procedure, an increased rate of withdrawal or retraction of the end of a bag is necessary in order to preserve the integrity of the end of the bag adjacent to the super heated cutting edge.
Advancement of the cutting edge and rapid retraction of the thermoplastic web during cycle interrupt has been difficult to achieve due to constraints associated with the air cylinder and other mechanisms employed in present bag making machines. These limitations inhibit the benefits associated with a self-cleansing heated cutting edge and a rapid bag making cycle.
The difficulties suggested in the preceeding are not intended to be exhaustive but rather are among many which may tend to reduce the effectiveness and product quality associated with present bag making machines. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that thermoplastic bag making machines appearing in the past will admit to worthwhile improvement.